Many religions throughout history have achieved explosive growth rates, but none so far as I know have done significant experimentation to optimize their methodology, and

I'd guess that most widespread religions have pretty good methodology for getting spread, because they were selected for effectively spreading. Like, every major religion started out smaller, and we don't often hear about the ones that didn't make it big.

a lot of religious conversion tactics rely on what we consider here to be Dark Arts, which are not nearly as conducive to encouraging people to become rationalists.

Is this true?

Like, I know the Dark Arts would be bad for rationalists to exploit, but I'm not sure that it would necessarily be less effective at introducing people to rationality.